Ethylene is produced by passing a feedstock containing naphtha and other distillates through a furnace comprised of a series of tubes. To achieve desired creep strength and oxidation resistance, these tubes are made of higher alloys such as the wrought Alloy 800 series and centrifugally cast alloys such as HP, HK, and HH alloys. The feedstock enters the furnace at a temperature of about 1000.degree. F. where it is heated to about 1650.degree. F. During the process pyrolytic coke is produced. Some of the coke accumulates on the walls of the furnace tubes. Nickel in the tubes reacts with the coke to form long whisker-like structures that extend from the walls of the tubes called catalytic coke. These strands tend to catch pyrolytic coke passing through the tubes to form a complex amorphous coke coating on the inner wall of the furnace tubes. This coating acts as an insulator reducing the temperature of the inner walls of the furnace tubes. Consequently, the furnace must be periodically cleaned to remove this coating. This cleaning is often called decoking. At many locations the tubes must be cleaned every six weeks.
The art has attempted to control catalytic coking by the selection of high chromium, high silicon content alloys or by applying a chromium or aluminum or ceramic coating to the inner walls of the furnace tube. However, higher chromium introduces instability in the alloy structures. Aluminum coatings have found limited success on wrought alloys with process temperatures not exceeding 1600.degree. F. At higher temperatures inter-diffusion and spalling occurs. Ceramic coatings suffer from cracking and spalling.
Coatings of two or more materials have also been proposed for metals used in high temperature process applications. In Japanese Patent 80029151 there is disclosed a method for applying a chromium-aluminum-silicon coating. This coating is produced by a chromium pack cementation process followed by an aluminum-silicon pack cementation process. The coated metal is said to be useful for jet engines, gas turbines and internal combustion engines. In U.S. Pat. No. 3,365,327 there is disclosed a method for vapor diffusion coating metallic articles with aluminum-chromium-silicon to provide elevated temperature corrosion resistance for gas turbine and oil refinery applications. In U.S. Pat. Nos. and 4,500,364 and 4,310,574 there are disclosed methods for applying an aluminum-silicon coating for high temperature process applications. The technique involves a slurry coating followed by high temperature firing. There is no teaching in any of these references that such coatings would be useful for ethylene furnace tubes.
Pack cementation is a well known technique for applying diffusion coatings to metal surfaces. This process involves placing a pack mixture into close contact with the surface being coated and subsequently heating the entire assembly to an elevated temperature for a specified period of time. During heating the coating material diffuses from the pack onto the surface of the metal. A common pack mixture used to create a chromium coating contains chromium, an inert filler such as alumina, and a halide activator such as ammonium chloride. The pack cementation process is particularly useful for coating inner walls of tubular structures. However, prior to the present invention the art has not created a pack cementation process that significantly reduced the formation of catalytic coke deposits on the inner walls of ethylene furnace tubes.
The art has also proposed co-diffusing chromium and silicon, chromium and aluminum, or aluminum and silicon in a single step pack cementation process. These methods have several disadvantages including difficulty in obtaining process control of the diffusion coating composition and nonuniform diffusion coating thickness on large scale due to pack heat transfer limitations. Due to the temperature gradients found in large powder packed retort, laboratory processes are usually difficult to scale-up to commercial processes in a manner which provides for diffusion coating thickness and composition uniformity on large components.
Whenever a metal alloy containing nickel, chrome and iron is coated using a diffusion process, a nickel and iron-rich overlay is formed on the coating. In the past no effort was made to remove this overlay. However, we have discovered that the overlay promotes coking when present on ethylene furnace tubes.
Consequently, there is a need for a effective method of treating high alloy ethylene furnace tubes to reduce catalytic coking.